Significant increases in the storage densities of magnetic disc media have resulted from improvements in recording techniques and improvements in the magnetic media. Regarding the latter, the most significant recent improvement has been the so-called thin-film disc where one or more plated (or sometimes sputtered) metallic layers replace the spin-coated particulate (iron oxide) layer used in earlier discs. The metallic layers often define a composite structure including a non-magnetic (for example chromium) layer and a magnetic (for example cobalt) layer. Such thin-film media are more durable, have better signal-to-noise ratios, and should ultimately prove to be cheaper than the particulate media.
Regardless of the particular coating technology, a suitable substrate is required to receive the subsequent layers. In the fabrication of commercial thin-film discs, aluminum is nearly universal. A layer of non-magnetic nickel alloy (such as nickel-phosphorus) is deposited on a polished aluminum base by an electroless plating process to a thickness on the order of 8-20 microns. The nickel alloy surface is then polished flat (say to a roughness less than 250.ANG. or 1 microinch) using a very fine abrasive to provide a finished layer having a thickness on the order of 5-15 microns. The plated and polished structure provides the finished substrate which is then subjected to further plating or sputtering steps to deposit the magnetic layer. A wear-resistant overcoat is normally deposited over the magnetic layer. In view of its position within the finished disc, the nickel alloy layer is referred to as the undercoat.
There is, of course, no requirement that the substrate fabrication and the magnetic coating be done in the same facility or by the same entity. Indeed, it is more typical for the substrates to be fabricated by one manufacturer and supplied to another for magnetic coating and testing.